Flyback converter with wake-up circuitry

ABSTRACT

A circuit arrangement is disclosed for controlling the switching of a field effect transistor (FET). A current controlled amplifier may be configured to amplify a current in a current sense device to generate an amplified current, wherein the current in the current sense device indicates a current through the FET. A comparator may be coupled to the current sense amplifier to compare a voltage corresponding to the amplified current with a voltage reference and to generate a comparator output based on the comparison, wherein the comparator output controls whether the FET is on or off.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/292,643, filed Oct. 13, 2016, which is a continuation of U.S. patentapplication Ser. No. 14/553,108, filed Nov. 25, 2014, which issued asU.S. Pat. No. 9,473,132 on Oct. 18, 2016, which claims the benefit ofU.S. Provisional Application Ser. No. 61/908,583, filed Nov. 25, 2013,the contents of which are hereby incorporated by reference herein.

FIELD OF INVENTION

The disclosure relates to the control of a switch. More particularly,the disclosure relates to the control of a synchronous field effecttransistor (FET).

BACKGROUND

Synchronous (sync) FET controllers traditionally control the sync FET inthe voltage domain. A traditional sync FET controller senses a forwardVf voltage drop of a body diode of the sync FET to determine a point atwhich to turn on the sync FET. The sync FET further senses a voltageacross the Rds_on of the sync FET to determine a point at which to turnoff the sync FET.

A voltage controlled amplifier (VCA) may amplify a voltage across theRds_on of the MOSFET, and the amplified voltage may then be used tocontrol when to turn off the sync FET. Sensing and controlling the syncFET in the voltage domain is susceptible to influences due to parasiticinductors (L's) and capacitors (C's) of the circuit layout and the partsinvolved. The influence of the parasitic L's and C's may degrade theability to accurately control the switching on and off of the sync FET.

Voltage domain sync FET control may be inaccurate when sensing signalswhich have a high change in current vs. time (dI/dt), such as near thezero crossing of an LLC waveform. Parasitic L's and C's in combinationwith the high dI/dt can cause cross conduction due to propagation delaysin the VCA. At light loads, when the traditional sync FET controllersenses the forward Vf of the body diode to turn on the sync FET and Rdson to turn off the sync FET, the traditional sync FET control mayoperate inefficiently. In the load light load condition, after thetraditional controller turns on the sync FET based on the forward Vf ofthe body diode, the controller may then quickly turn off the sync FETbased on the voltage detected across the Rds_on of the sync FET being ator below a turn-off threshold. As such, the traditional sync FETcontroller introduces additional losses due to the unintentionalswitching of the sync FET plus the loss from the body diode without thebenefit of using the low Rds_on of the sync FET.

Thus, there exists a need for a more accurate and efficient sync FETcontroller.

SUMMARY

A sync FET controller is disclosed which uses current domain control toeffect high speed control of the turn on/off points for the gate driveof the sync FET controller. The sync FET controller may use the copperon the printed circuit board (PCB) or a shunt resistor as a currentsense element to sense the current flowing between output capacitors anda transformer. The thermal drift of the copper on the PCB may also becompensated. A method and circuit arrangement is disclosed for settingthe gain and turn on/off points of the sync FET controller. Further, amethod and circuit arrangement is disclosed for providing a wakeup pulsefrom a secondary side back to a primary side for deep sleep modes ofoperation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an example embodiment of a controller.

FIG. 2 depicts an example of waveforms that may be associated with thewake-up feature of an embodiment of a controller.

FIG. 3 depicts a schematic of an example current controlled amplifier(CCA) which may be used in an embodiment of a controller.

FIG. 4 depicts a schematic of another example CCA which may be used inan embodiment of a controller.

FIG. 5 depicts a schematic of an example implementation of a voltagecomparator for gate drive generation.

FIG. 6 depicts a schematic of an example implementation of a voltagecomparator for a wake-up feature.

FIG. 7 depicts a schematic of an example flyback converter including anembodiment of a controller.

FIG. 8 depicts a schematic of an example LLC converter including anembodiment of a controller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Disclosed herein is a controller of a switch that may be used, forexample, in discontinuous conduction mode (DCM) power converters. Theswitch may be a FET in a DCM power converter. Examples of powerconverters in which the controller may be used include, but are notlimited to, a quasi-resonant (QR) converter, a QR flyback converter, andan LLC power converter. In said converters, the FET may be switched onand off to effect the conversion of power from the input to the output

An example embodiment of an FET controller 100 is depicted in FIG. 1.The FET may be a switch Q1 110, wherein a gate of the switch Q1 110,which determines whether the switch is closed/on or open/off, iscontrolled by the GATE DRIVE. The switch Q1 110 may comprise a metaloxide semiconductor (MOS) FET, which may be an NMOS FET or PMOS FET. Theswitch may alternatively be another type of transistor, such as abipolar transistor.

The switch Q1 110 may be coupled between a secondary side of atransformer (not depicted) and an output capacitor Co 190. The outputcapacitor Co 190 may store a charge at the output of a power converter.When the switch is closed/on, a current may flow through the switch Q1110 to charge the output capacitor Co 190. The timing of the switchingof the switch Q1 110 between being closed/on and open/off may controlthe conversion of power into stored energy/charge in the outputcapacitor Co 190 for delivery to a load. A voltage Vo may be built upacross the output capacitor Co 190.

In an embodiment of a controller 100 configured in accordance with theteachings herein, the controller 100 may include a current controlledamplifier (CCA) 120. The CCA 120 may include two inputs, for example anegative current input SEN− 122 and a positive current input SEN+ 121 asdepicted in FIG. 1. The CCA 120 may take a differential current betweenthe two current inputs and amplify the differential current by a currentgain to generate an amplified current output. The CCA 120 may furthercomprise a plurality of current controlled amplifiers. The inputs may becoupled to the switch Q1 110 or to a current sense device R_(CS) 101 tosense a current flowing through the switch Q1 110.

The current sense device R_(CS) 101 may be coupled to the switch Q1 110.Current sense device R_(CS) 101 as depicted in FIG. 1 may comprise ashunt resistor or a copper trace coupled to the switch Q1 110. Thecurrent input SEN− 122 may be coupled to a first node of the currentsense device R_(CS) 101 via a resistor R_(G) 102. The current input SEN+121 may be coupled to a second node of the current sense device R_(CS)101. The second node may be the ground node of the power converter ormay be another power supply node. The CCA 120 may have, for example, acurrent gain of approximately 1000. The current gain of the CCA 120 maybe controllable to tune a turn-on point at which the switch Q1 110 isturned on and to tune a turn-off point at which the switch Q1 110 isturned off. The CCA 120 may be, for example, a modified Wilson currentamplifier, but the CCA 120 is not limited to any particular topology.Biasing may be added to the Wilson current amplifier to improve itsspeed.

In an embodiment of a controller 100 configured in accordance with theteachings herein, wherein the current sense device R_(CS) 101 is acopper trace, temperature compensation may be implemented to compensatefor a drift in the resistance of the copper trace due to a change inoperating temperature. A drift in an explicit resistance may also becompensated. A negative temperature coefficient (NTC), a positivetemperature coefficient (PTC), a PN junction, or any combination thereofmay be used to compensate for said resistance drift. Furthermore, avoltage controlled current may be sunk from an appropriate node orsourced to the appropriate node to compensate for changes in theresistance due to a change in temperature. Additionally oralternatively, a voltage may be modified to compensate for changes inthe resistance due to a change in temperature. Placement of thecomponents configured to perform temperature compensation may bedependent upon the particular topology implemented. Examples ofcompensation configurations will be described in more detail below.

In an embodiment of a controller 100 configured in accordance with theteachings herein, a resistance R_(GTRIP) 106 may be coupled to theoutput of the CCA 120. The output of the CCA 120 may be further coupledto first input 141 of a voltage comparator V_(COMP2) 140. A voltagereference Vref may be coupled to a second input 142 of the voltagecomparator V_(COMP2) 140. The resistor R_(G)TRIP 106 may be sized to seta current output from the CCA 120 at which the voltage comparatorV_(COMP2) 140 turns on. This turn-on current may be referred to asI_(SET) as depicted in FIG. 1. The CCA 120 current output flowingthrough resistor R_(GTRIP) 106 may induce a voltage at the first input141 of the voltage comparator V_(COMP2) 140. When the induced voltage atthe first input 141 exceeds Vref, the output of the voltage comparatorV_(COMP2) 140 may turn on. The voltage comparator V_(COMP2) 140 mayalternatively be configured to operate such that when the inducedvoltage at the first input is less than Vref the output of the voltagecomparator turns on.

In an embodiment of a controller 100 configured in accordance with theteachings herein, the output of the controller GATE_DRIVE may be coupledthrough a resistance R_(HYST) 105 to the first input 141 to create ahysteresis band around the voltage comparator V_(COMP2) 140 to set aturn-off point of the voltage comparator V_(COMP2) 140. Alternatively,the voltage comparator V_(COMP2) 140 may be configured by a separateturn-off point.

In an embodiment of a controller 100 configured in accordance with theteachings herein, gate drive circuitry may be coupled between the outputof the voltage comparator and a gate of the switch Q1 110. FIG. 1depicts an example of gate drive circuitry including an example drivingtransistor pair of a PNP 182 and NPN transistor 181.

In an embodiment of a controller 100 configured in accordance with theteachings herein, the controller 100 may include circuitry configured toenable the controller 100 to enter into a deep sleep operation modeduring light or no load conditions. In the deep sleep mode, thecontroller 100 may shut down the gate drive circuitry and other unuseddigital circuitry that may be draining power. In deep sleep mode, theprimary side of the converter may also enter a sleep mode which may alsosave power by shutting down unused circuitry on the primary side of thetransformer. The controller 100 may further include circuitry configuredto generate a wake-up pulse to the primary side in response to anincrease in a load demand or in response to an output voltage droppingto a minimum threshold voltage. The ability to provide the wake-up pulsemay reduce a quantity of parts needed to implement the converter and mayalso improve a load step response.

As depicted in FIG. 1, resistors R_(WAKEUP1) 103 and R_(WAKEUP2) 104 maybe configured as a voltage divider to set Vminth, which is a function ofthe output voltage, for triggering a wake-up pulse. The voltagecomparator V_(COMP1) 130 may compare the threshold voltage Vminth at afirst input 131 to a reference voltage Vref coupled to a second input132. When Vminth drops below Vref, the voltage comparator V_(COMP1) 130may turn on to trigger a monostable 150, which may also be known as aone-shot, to deliver a pulse to drive the GATE_DRIVE. The GATE_DRIVE maythen cause switch Q1 110 to pulse by switching on and off. The generatedpulse may be fed back to the primary side of the power converter throughthe transformer which may be detected, for example, by an auxiliarywinding on the transformer to wake up the primary side. The primary sidemay then cause the secondary side, including the controller 100, to wakeup and return to synchronously switching the switch Q1 110. Thecircuitry for generating the wake-up pulse may be enabled or disabledand may be, for example, enabled by default. More details regarding theoperation of the wake-up circuitry will be provided below. Furthermore,FIG. 1 depicts a Blanking Generator 160 coupled to the monostable 150,the function of which will be described in detail below. Though thecontroller 100 depicted in FIG. 1 shows wake-up circuitry includingV_(COMP1) 130, R_(WAKEUP1) 103, R_(WAKEUP2) 104, monostable 150,Blanking Generator 160, and an OR gate 170, such circuitry may also beleft out of a controller 100 so that the wake-up feature is notimplemented.

The configuration of the CCA 120 and the voltage comparator V_(COMP2)140 enables accurate sensing of low currents in the switch Q1 110. Theconfiguration may be further capable of high speed control of theswitching of the switch Q1 110. Such high speed control may be a directbenefit of amplifying in the current domain via the CCA 120 up until thevoltage comparator V_(COMP2) 140, in contrast to amplification in thevoltage domain performed by traditional controllers. By amplifying inthe current domain, the controller 100 is less susceptible to parasiticinductances and capacitance that may slow response time and induceringing which may impede accurate control of the switch Q1 110. Theturn-on/turn-off points and hysteresis of the controller 100 may beadjustable so that a propagation delay from sensing current to a drainon the switch Q1 110 may be minimized and losses associated with theforward Vf of the switch Q1 110 may also be minimized.

The disclosed configuration of the CCA 120 and the voltage comparatorV_(COMP2) 140 may enable turning on and off the switch at lower levelsso that the switch may conduct current during more of a total switchingperiod than conventional means, thus reducing losses caused by bodydiodes. Furthermore, the controller 100 may exhibit improved low or noload efficiency because the controller 100 may not rely upon Rds on likeconventional controllers which causes conventional controllers toinefficiently turn on and off repeatedly in such low or no loadconditions.

In an embodiment of a controller 100 configured in accordance with theteachings herein, the controller 100 may include gate drive limitingcircuitry configured to selectively limit an output of the voltagecomparator V_(COMP2) 140 and/or gate drive circuitry. The output may belimited to a selected voltage, for example the output may be limited toeither 5 volts or 10 volts.

In an embodiment of a controller 100 configured in accordance with theteachings herein, the controller 100 may include under voltage lockout(UVLO) circuitry to sense a supply voltage. The UVLO circuitry mayprevent operation of the controller 100 when the supply voltage dropsbelow a selectable threshold. For example, the threshold may be selectedto be 4 volts or 9 volts. The UVLO circuitry may be enabled or disabledand may be, for example, enabled by default.

A controller 100 configured in accordance with the teachings herein maybe manufactured in a small package, for example an SOT23-6 package. Thesmall package may enable the controller 100 to be implemented in systemswith tight size and space requirements.

The principles disclosed herein may be applied generally to currentsensing applications to eliminate current sensing resistors. Forexample, the CCA 120 and voltage comparator V_(COMP2) 140 arrangementsuch as that depicted in FIG. 1 and described above may be used to senseinputs for a pulse width modulation (PWM) controller for a power factorcorrection (PFC) stage. The current sensing topology may also be appliedto other DC/DC conversion applications or as part of an over-currentdetector.

Although the resistances in FIG. 1 are depicted as explicit resistors,one should recognize that any suitable component may be used whichprovides a resistance. For example, a depicted resistor may be replacedby a suitable bipolar transistor, FET transistor, diode, capacitor,inductor, or any combination thereof. Furthermore, the depictedresistances may be external resistances or may be incorporated withinthe controller package. Furthermore, although FIG. 1 may depict thecontroller 100 coupled to ground and to a single positive voltagesupply, the controller 100 may be alternatively or additionally coupledto one or more positive voltage supplies and/or one or more negativevoltage supplies. Moreover, one should recognize that the controllerconfiguration may further include additional circuitry such as buffersand additional connections to external circuitry based on the particularimplementation.

Although the controller 100 depicted in FIG. 1 is configured with aswitch Q1 110 coupled to ground, and the CCA 120 is further coupled tothe ground side to sense the current through the switch Q1 110, one ofskill in the art should recognize that an inverted configuration suchthat the switch Q1 110 is coupled to Vo may also be possible. In such aconfiguration, a PMOS or a charge pump in combination with an NMOS maybe coupled on the Vo side for a high-side switched sync FET controllerarrangement. In such a high-side switched arrangement, the current maybe sensed through the switch Q1 110 coupled to Vo and amplified by anappropriately inverted version of CCA 120 configured to sense andamplify said current.

FIG. 2 depicts an example of waveforms that may be associated with thewake-up feature of an embodiment of the controller 100 described above.Vo may be an example output voltage across the output capacitor Co 190.This voltage Vo may also be referred to as Vcc. The GATE_DRIVE waveformmay be a voltage waveform associated with the depicted Vo voltagewaveform. The switching of the GATE_DRIVE may charge the outputcapacitor to the output voltage Vo. Once the output voltage Vo is fullycharged and the switching stops in the GATE_DRIVE, a blanking period maybe implemented such that the monostable 150, also known as a one-shot,may be disabled so that it cannot deliver a pulse during the blankingperiod. The Blanking Generator 160 depicted in FIG. 1 may be thecircuitry coupled to the monostable 150 to prevent the monostable 150from outputting a pulse during the blanking period. The BlankingGenerator 160 may output an enable/disable signal to the monostable 150.The blanking period may be a programming period of, for example, 1 ms to10 ms in length. The blanking period may ensure that the monostable 150does not fire during a time that could effectively short the primaryside to the secondary side.

As described with respect to FIG. 1, Vminth is a function of Vo throughthe voltage division of R_(WAKEUP1) and R_(WAKEUP2). When Vminth reachesVref, voltage comparator V_(COMP1) of FIG. 1 may cause the monostable150 to deliver a pulse to the gate drive circuitry. As described abovewith respect to FIG. 1, the pulse from the monostable 150 may be thenfed back to the primary side to wake-up the converter, at which pointthe switching of the GATE_DRIVE may recommence to recharge the outputvoltage Vo across the output capacitor Co 190.

FIGS. 3 and 4 depict schematics of example CCA topologies which may beused as an implementation of the CCA 120 depicted in FIG. 1. FIG. 3depicts a CCA implemented using bipolar transistors 320 a-230 j and FIG.4 depicts a CCA implemented using FETs 420 a-420 j. The particularimplementation of the CCA 120 in FIG. 1 is not limited to only the CCAsdepicted in FIGS. 3 and 4. The resistances R_(G) 302/402, R_(CS)301/401, R_(HYST) 305/405, and R_(GTRIP) 306/406 depicted in FIGS. 3 and4 correspond with the similarly labeled resistances in FIG. 1. Thedepicted CCAs comprise a Wilson current amplifier further configuredwith voltage biasing through resistance R_(BIAS) 308/408. FIGS. 3 and 4further depict optional thermistors NTC1 351 a/451 a, PTC1 351 b/451 b,and NTC2 352/452. Any one thermistor or combination of these thermistorsmay be included to compensate for the change in resistance due to achange in temperature as described above with respect to FIG. 1.Although thermistors are depicted, a PN junction may be used. The effectof the changing resistance in the thermistor is to modify the voltageand current at the nodes to which it is coupled. As described above,additionally or alternatively a voltage controlled current may be sunkor sourced to compensate for changes in the resistance due to a changein temperature. Additionally or alternatively, a voltage may be modifiedto compensate for changes in the resistance due to a change intemperature.

FIG. 5 depicts a schematic of an example implementation of a voltagecomparator for gate drive generation, such as the voltage comparatorV_(COMP2) 140 depicted in FIG. 1. The voltage comparator V_(COMP2) 140of FIG. 1 is not limited to the topology depicted in FIG. 5. TheV_(COMP2) input 541 depicted in FIG. 5 may serve as the positive input141 of V_(COMP2) 140 depicted in FIG. 1 coupled to the output of the CCA120. The other input 542 of the voltage comparator depicted in FIG. 5may serve as the negative input 142 of V_(COMP2) 140 depicted in FIG. 1coupled to Vref.

FIG. 5 further depicts optional thermistors NTC1 553 a and PTC1 553 b.Any one or combination of these thermistors may be included tocompensate for the change in resistance due to a change in temperatureas described above with respect to FIG. 1. Although thermistors aredepicted, a PN junction may be used. The effect of the changingresistance in the thermistor is to modify the voltage and current at thenodes to which it is coupled. As described above, additionally oralternatively a voltage controlled current may be sunk or sourced tocompensate for changes in the resistance due to a change in temperature.Additionally or alternatively, a voltage may be modified to compensatefor changes in the resistance due to a change in temperature.

FIG. 6 depicts a schematic of an example implementation of a voltagecomparator for the wake-up feature described above, such as the voltagecomparator V_(COMP1) 130 depicted in FIG. 1. The resistances R_(WAKEUP1)603 and R_(WAKEUP2) 604 depicted in FIG. 6 correspond with the similarlylabeled resistances in FIG. 1. As similarly shown in FIG. 1, Vminth maybe coupled to the one input 631 of the voltage comparator depicted inFIG. 6, and Vref may be coupled to the other input 632.

FIG. 7 depicts a simplified schematic of an example embodiment of acontroller in an example flyback converter. The controller maysynchronously control the switching of switch Q1 710 to charge theoutput capacitor Co 790 to provide an output voltage Vo. FIG. 7 furtherdepicts an auxiliary winding 715 of the transformer which may sense thewake-up pulse generated by the wake-up circuitry to wake the flybackconverter from a deep sleep mode in accordance with the descriptionprovided above.

FIG. 8 depicts a simplified schematic of an example embodiment of acontroller in an example LLC converter, wherein two controllers areimplemented. The controllers may control the respective switches Q1 810a and 810 b to charge the output capacitor Co 890 to provide an outputvoltage Vo. FIG. 8 further depicts an auxiliary winding 815 of thetransformer which may sense the wake-up pulse generated by the wake-upcircuitry to wake the flyback converter from a deep sleep mode inaccordance with the description provided above.

While the various embodiments described and depicted may be shown by wayof example using a particular topology or a particular device, one ofordinary skill in the art recognizes that alternatives fall within thespirit of teachings disclosed herein. For example, the schematics shownin FIGS. 5, 6, and 7 depict the use of bipolar transistors, but one ofordinary skill should readily recognize that FETs or other suitablenon-linear devices may be used. Likewise, for example, while a resistormay be depicted, one of ordinary skill will readily understand that adiode, transistor, capacitor, inductance, or any other device suitablefor providing resistance may be used.

While various embodiments of the disclosed controller have beendescribed in detail, further modifications and adaptations of theembodiments may be apparent to those skilled in the art. It should beunderstood that such modifications and adaptations are within the spiritand scope of the present disclosure.

What is claimed is:
 1. A flyback power converter comprising: atransformer including a primary side and a secondary side; an outputcapacitor coupled to the secondary side of the transformer, the outputcapacitor configured to store an output charge; a switch seriallycoupled between the second side of the transformer and the outputcapacitor, wherein the switch is configured to allow current to flow tocharge the output capacitor when the switch is closed and to preventcurrent to flow when the switch is open; a controller coupled to thesecondary side of the transformer, the controller configured to controlwhether the switch is open or closed, wherein the controller includes acurrent controlled amplifier to amplify a sensed current flowing throughthe switch; and a wake-up circuit configured to deliver a pulse to wakeup the flyback power converter.
 2. The flyback power converter of claim1, wherein a gain of the current controlled amplifier is controllable totune a turn-on point at which the switch is turned on and to tune aturn-off point at which the switch is turned off.
 3. The flyback powerconverter of claim 1, wherein the switch is a field effect transistor(FET).
 4. The flyback power converter of claim 3, wherein the controllerincludes gate drive circuitry configured to drive a gate of the FETbased at least in part on the sensed current flowing through the FET. 5.The flyback converter of claim 4, wherein the wake-up circuit is furtherconfigured to deliver the pulse to the gate drive circuitry.
 6. Theflyback converter of claim 5, wherein the gate drive circuitry isfurther configured, in response to the pulse, to drive the gate of theFET to cause the FET to switch on and off.
 7. The flyback converter ofclaim 6, wherein the primary side of the transformer includes anauxiliary winding.
 8. The flyback converter of claim 7, wherein the FETswitching on and off is sensed on the auxiliary winding of the primaryside of the transformer.
 9. The flyback converter of claim 4, whereinthe controller is formed in a
 10. The flyback converter of claim 1,wherein the wake-up circuit is configured to feed back the pulse inresponse to a voltage dropping below a threshold.
 11. The flybackconverter of claim 10, wherein the voltage is a based on an outputvoltage across the output capacitor.
 12. The flyback converter of claim11, wherein the output voltage across the output capacitor decreasesbelow the threshold due to an increase in a load demand.
 13. The flybackconverter of claim 10, wherein the flyback converter is configured tooperate in a deep sleep mode of operation during a light load or no loadcondition.
 14. The flyback converter of claim 13, wherein the pulse isused to wake the flyback converter from the deep sleep mode ofoperation.
 15. The flyback converter of claim 13, wherein the controlleris configured to shut down gate drive circuitry.
 16. The flybackconverter of claim 15, wherein unused circuitry on the primary side ofthe transformer is shut down during the deep sleep mode of operation.17. The flyback converter of claim 10, wherein the wake-up circuitincludes a monostable configured to generate the pulse.
 18. The flybackconverter of claim 17, wherein the monostable is configured to beselectively enabled/disabled.
 19. The flyback converter of claim 18,wherein the wake-up circuit further includes a blanking generator,wherein the blanking generator provides a signal to selectivelyenable/disable the monostable.
 20. The flyback converter of claim 19,wherein the blanking generator is configured to prevent the monostablefrom generating the pulse during a time that would effectively short theprimary side of the transformer to the secondary side of thetransformer.